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# Dew Point Mirror

## Instrumentation

The BCO is equipped with a "Ventilated Thermohygrometer VTP6 Thygan" manufactured by Meteolabor AG. The instrument combines a ventilated and heated thermometer with a mirror-type dew point hygrometer and has been measuring since 2021-07-08.


```{figure} VTP6_Thygan.png
:alt: Dewpoint mirror
:width: 400px
:align: center

The Thermohygrometer VTP 6 with the the air intake visible at the bottom. The air exits the housing through a duct below the lid.
```

The instrument is installed $29\,\mathrm{m}$ above mean sea level.

The measurable temperature range is -50 to +50 °C, while the dew point range is -65 to +50 °C. Every ten minutes, the device takes about ten measurements of air temperature and dew point temperature within a 40-second interval. The device provides the mean values along with their standard deviations, allowing data quality to be assessed. The temperature resolution is 0.1 K, and the relative humidity resolution is 0.1%.

The program evaluates three types of device messages: measured values, status message, and raw data. These messages contain the following 17 variables:

| Abbreviation | Variable | Device message | Unit |
|---|---|---|---|
| ST | Measurement status | Measured values | - |
| TT | Air temperature | Measured values | degree_Celsius |
| DT | Dew point temperature | Measured values | degree_Celsius |
| RH | Relative humidity | Measured values | % |
| RTT | Air temperature | Raw data | degree_Celsius |
| RSTT | Standard deviation of air temperature | Raw data | degree_Celsius |
| RDT | Dew point temperature | Raw data | degree_Celsius |
| RSDT | Standard deviation of dew point temperature | Raw data | degree_Celsius |
| RRH | Relative humidity | Raw data | % |
| TG | Housing temperature | Raw data | degree_Celsius |
| VU | Fan voltage | Raw data | V |
| VI | Fan current | Raw data | mA |
| K | Value | Raw data | - |
| RST | Measurement status | Raw data | - |
| OA | Option 0 | Status message | - |
| OB | Option 1 | Status message | - |
| SW | Status byte W3 | Status message | - |

## The measurement principle

The air temperature is measured by a copper-constantan thermocouple. To measure the dew point temperature, moist air is passed over a mirror. The mirror is cooled until the dew point temperature is reached and condensation occurs. To detect the optical change, a light beam is directed onto the mirror and reflected by it. The intensity of the reflected light beam decreases when condensation forms on the mirror. This reduction can be detected by an optical system.

## Data Availability

```{code-cell} python
import intake

cat = intake.open_catalog("https://tcodata.mpimet.mpg.de/catalog.yaml")
ds_dew = cat.BCO.dewpoint.to_dask()
ds_dew
```

## Sample Plot

```{code-cell} python
ds_dew.DT.sel(time="2025").resample(time="1D").mean().plot(label="Dew point")
ds_dew.TT.sel(time="2025").resample(time="1D").mean().plot(label="Air temperature")
```